Drills may be broadly categorized as twist drills, that is, those in which the side flutes have a helical twist, and gun drills, in which the side flutes are straight. The twist drill-gun drill distinction is not relevant to differences in the cutting end of the drill, however, which is what the subject invention concerns. Therefore, only twist drills need be considered, which will be referred to simply as drills per se.
Drills may be further categorized as center cutting drills and non center cutting drills, also known as core drills. Non center cutting drills act to widen a smaller diameter pilot hole that has already been provided, generally by first drilling it with a smaller diameter, less accurate center cutting drill. Non center cutting drills can easily incorporate multiple flutes and their associated cutting edges, since the cutting edges need not be complete. That is, they need not run all the way in to the central axis of the drill, because the drill does not have or need a cutting point. Given the multiple flutes and the multiple lands between the flutes, non center cutting drills pilot in the holes that they are cutting with better stability, giving superior accuracy and straightness. Multiple pass drilling is inherently slower and more expensive than single pass drilling, of course.
Center cutting drills have two axial side flutes are separated by a pair of diametrically opposed lands. The center cutting point is not a true, sharp point, but a pair of short chisel edges that radiate out from the center axis to two cutting edges or lips, which are parallel to either side of a bisecting plane. The chisel edges are formed by the line of intersection of a lead facet on one side of the point, which creates the cutting edges, with a trailing clearance flank on the opposite side, which allows the cutting edges to clear the workpiece as they cut. The chisel edges, formed as they are, resemble the center ridge of a roof, and cannot be made sharp in the sense that the cutting edges of the drill can be made sharp. The chisel edge is also the most slowly moving part of the drill, being nearest to the center. This combination of inherent dullness and slow speed means that the chisel edges do not so much cut a chip as they plow up or extrude a chip ahead of them. This extruded workpiece material tends to build up in front of the chisel edge, wearing it more quickly than the faster moving and sharper main cutting edges.
In order to aid in chip formation and removal, each of the two flutes is provided with a ground trough known as a gash. These curl up the chips formed in front of the cutting edges, and help break them up and send them down the flutes and ultimately out of the hole. This process may be aided by a pair of coolant holes, one drilled through each land just ahead of the chip removal gash. Highly pressurized coolant pumped down the holes flushes the chips off the cutting edge and out. In a two flute drill, each chisel edge forms an obtuse angle with the cutting edge that precedes it, and has no overlap with it. Instead, each chisel edge lies totally behind the cutting edge that precedes it, next to a chip removal gash. This configuration provides an easy exit path for the material plowed up ahead of the chisel edge, which can flow down the clearance flank of the leading cutting edge and into the adjacent gash. Two flute drills are inherently less stable than a multiple flute drill, however, because there are fewer lands to ride inside the hole being drilled. Consequently, holes drilled with two flute, center cutting drills are generally less accurate.
While multiple flute drills are understood to be more stable and accurate than two flute drills, center cutting drills with more than two flutes are not known to exist commercially. In fact, in the twist drill art, the term two flute drill has become synonymous with center cutting drill, and the term three or four flute drill has become synonymous with core drill, for that very reason. While one might think that a workable center cutting drill with three or four flutes could simply be made as a multiplied, scaled up version of a two flute drill, with three or four complete cutting edges meeting somehow at the center, that is not the case. If that were attempted, the center point where the edges met Would look like a three or four sided pyramid. The leading edge of each side of the pyramid would constitute a chisel edge. The chisel edges so formed would now form an acute angle with the cutting edges preceding them, and would not lie totally behind. In addition, the chisel edges would have very little overlap with any chip removal gash, and there would be no easy exit path for the chisel edge chips that were formed. The chisel edge chips formed would have to fight their way down the shallowly sloped lead facet of the preceding cutting edge, rather than down the more steeply sloped clearance flank, before they could reach the chip gash. Consequently, the chisel edge chips would tend to build up, the axial load tending to thrust the drill out of the hole would be large, and the wear rate of the chisel edges high. Experimentation has shown this.